Sounds like the dealers lost the plot.
The oil level and pressure aren't related.
Obvioulsy if the level got too low you'd lose pressure.
But theres a seperate sensor for the oil level to the oil pressure.

So it sounds like the oil, the filter and the oild pressure are just fine.
The oil level sensor maybe at fault.
However if you visually check the level and it's ok don't worry about the level warning.

It must be the oil level sensor. I had exactly the same argument with Fiat including their headquarters when I had a Regata. It was only a few weeks old and the oil level light would come on when the car was moving. They kept telling me that the car must be low on oil even though it was not. I could not get through to them that the oil level sensor could not determine the level of the oil when it was being flung around the engine. That was the job of the oil pressure sensor. The camber of the road can effect the oil level sensor if it is not in the centre the engine. On the Regata it was to one side and i'm sure the Ulysse is also off centre as the sensor reads differently depending on the degree of that camber. Obviously if you have checked with the dip stick then the sensor must be faulty.

Lou